Petrology and P–T path of blueschists from central Qiangtang, Tibet: Implications for the East Paleo-Tethyan evolution

Abstract

The Paleo-Tethys Ocean records the initial geodynamic processes related to the dispersion of Gondwana and the subsequent accretion of Laurasia. However, whether the blueschist-bearing central Qiangtang metamorphic belt (CQMB) within the Qiangtang Block represents an in situ Paleo-Tethys suture remains controversial. Since the starting point of this debate is the subduction type (i.e., warm subduction vs. cold subduction) recorded by the associated central Qiangtang blueschists, here, we approach this debate by investigating their petrogenesis, which records the subduction and exhumation of an oceanic plate. We reconstruct the clockwise pressure–temperature (P–T) paths using phase equilibrium modeling for two types of blueschists from the Rongma and Gangma Co areas, and their peak P–T conditions reveal typical cold subduction with a uniform low geothermal gradient of 5–10 °C/km. Combining this information with published radiometric dating and bulk-rock geochemical data, we find that the central Qiangtang blueschists record almost all of the early Permian-Late Triassic subduction of the LSPTO and subsequent continental subduction, as well as the Late Triassic exhumation of oceanic and continental material. This study confirms that the central Qiangtang blueschist-bearing mélange marks an in situ suture, which formed during the northward subduction and subsequent exhumation of the LSPTO.